Info

Method

Procedure

Comments

von Stosch's method

Concentrate sample to near dryness, add equal amount of HNOj and 3X sample amount of H2S04, boil for ca. 3 min, cool, and rinse with distilled water until free of acid

Fast method, usually with good result

Simonsen's method

Rinse sample with distilled water, add an equal amount of KMnOj, agitate, leave for 24 hr, add an equal amount of HC1, heat until the sample becomes clear or only slightly colored, and rinse until free of acid

More time consuming, reliable

UV, H2Oj enzymes

Useful to obtain intact frustules

Difficult and time consuming

Note. Rinse and concentrate samples by settling or centrifugation at approximately 4000 rpm: small and lightly silicified valves and bands need long settling/centrifugation time; for settling, usually overnight; for centrifugation, up to half an hour.

Note. Rinse and concentrate samples by settling or centrifugation at approximately 4000 rpm: small and lightly silicified valves and bands need long settling/centrifugation time; for settling, usually overnight; for centrifugation, up to half an hour.

structures are best seen on single valves. This is a further reason to remove the organic material which keeps the various frustular elements together.

By acid cleaning (Table 80) the diatom frustule separates into single valves and bands free from organic material. Ultraviolet and enzyme techniques (Table 80) usually give intact frustules cleaned of organic material. The refractive index of silica and water is about the same, and to increase the contrast the cleaned material is embedded in a resin of a higher refractive index than that of silica (Table 81). The normal procedure thus includes (1) removal of the preservative by repeated centrifugation and decanting with distilled water, (2) cleaning, followed by (3) embedding in a mounting medium.

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